The invention concerns integrated glass optical components. The development of optical waveguide communication technologies has led to the design of new optical or opto-electronic components the function of which is to process optical signals leaving or entering optical fibers. Passive optical components can be distinguished from the active ones that include opto-electrical conversion relying on opto-electronic, opto-magnetic or opto-thermal effects.
Passive components include (a) connectors, which connect two fibers with minimum loss, (b) couplers which connect at least three fibers and split the power between the different fibers, and (c) couplers/multiplexers which split power between various fibers and select certain wavelengths.
Active components include the coupler/monitor which taps a small fraction of the power propagating in an optical fiber, the coupler/amplifier, the opto-electronic converter and switches or modulators. The first three types contain at least one "active element" such as a photodiode to ensure the opto-electrical conversion.
Components are also characterized according to whether single-mode or multimode propagation occurs in the optical path.
A large number of technologies have been proposed to make these components. Classical optical technologies have been employed, sometime down-scaled, to manufacture these active or passive components. Good performances have been achieved; however, these technologies are costly because the various elements have to be aligned with tolerances better than a micron, an alignment which is quite difficult to achieve.
For passive components, it is possible to use fiber fusion or fiber lapping technologies that allow coupling between fibers. These technologies, which are delicate to put into practice, do not apply broadly. It is difficult for instance to produce couplers with a large number of ports. Control of the coupling ratio is also difficult. Also, the integration of active components is practically impossible.
Another technology which has been proposed is planar integrated optics. This technology involves the generation of optical waveguides in a dielectric material by creating higher index zones for conducting light. The equivalent of an optical "printed circuit" can be made that way. Materials such as glass, Si, AsGa, or LiNbO.sub.3 have been proposed. Single-mode or multimode guides can be formed at the substrate surface. Such guides can be obtained by diffusion or implantation of ions which increase the index of the substrate (eg: K.sup.+, Ag.sup.+, Tl.sup.+ in glasses, Ti in LiNbO.sub.3) or by forming on the substrate layers having a larger index (Si, AsGa). Glasses are interesting because they are cheap and because it is easy to diffuse therein strong polarisability ions, thereby creating surface or buried waveguides. The diffusion process can take place using a molten salt bath technique well known in chemical tempering (see for instance: J. Goell and al. Bell System Tech. J. Vol. 48, p. 3445/3448 (1969) and H. Osterberg and al., J. of Opt. Soc. of America, vol. 54, p. 1078/1084 (1964). The molten salt bath diffusion technique offers the advantage of control of the refractive index of the guide by adjusting the strong polarisability ion concentration in the bath. Guide losses can be decreased by burying it below the glass surface. This can be accomplished by carrying out a second ion exchange step in a molten salt bath containing the ions initially present in the glass (or ions having lower polarisability) in the presence of an electrical field. The surface guide will migrate inside the substrate. U.S. Pat. No. 3,880,630 describes such a technology. This buried guide technology allows the fabrication of interesting components, couplers in particular, having good performance in the laboratory (see for example Nippon Sheet Glass technical report 1/1983 pages 3-6).
One of the most difficult problems to be solved for integrated optics is the mechanical positioning, with a tolerance of one micron or less, of fibers, sources or detectors on the integrated component. In the lab one can glue with epoxy resin the fiber on the component. It is a time consuming and delicate operation which is difficult to scale up industrially.
Optical fibers have been aligned with respect to an integrated optics circuit path by forming a groove in the substrate adjacent the end of the path. Such devices have had various disadvantages.
V-grooves can be accurately formed in silicon by crystallographic etching, a technique taught in U.S. Pat. No. 3,774,987. However, an optical waveguide path, to be effective, must be on a substrate or layer having a refractive index lower than that of the waveguide. Since silicon and similar materials that can be subjected to crystallographic etching have a high refractive index, they must be provided with a layer of low index material on which the waveguide path can be formed. In such a device the substrate material is expensive, and the process of adding a low index material to the surface further increases the cost.
U.S. Pat. No. 4,240,849 teaches that V-grooves can be formed in a plaastic substrate by molding or formed in a glass substrate by the so-called ultrasonic cutting method. Layers of plastic are then built up on the substrate to form an optical path that is aligned with optical fibers that have been cemented into the grooves. Such plastic materials result in lossy optical waveguides. Even if a glass optical path were deposited on the surface of a substrate, deposition techniques that must be employed result in relatively lossy waveguides, and such techniques are often limited to the formation of single-mode waveguides because of the minimal thickness of material than can be deposited. While grooves can be accurately positioned in a plastic substrate by molding, various properties of plastic render it undesirable for use as an optical waveguide material. The formation of grooves by ultrasonic cutting is too costly to be a commercially viable process, and grooves formed by that technique cannot be positioned with an accuracy of one micron.